Gas separation is important in many industries for removing undesirable contaminants from a gas stream and for achieving a desired gas composition. For example, natural gas from many gas fields can contain significant levels of H2O, SO2, H2S, CO2, N2, mercaptans, and/or heavy hydrocarbons that have to be removed to various degrees before the gas can be transported to market. It is preferred that as much of the acid gases H2S and CO2 be removed from natural gas as possible to leave methane as the recovered component. Small increases in recovery of methane can result in significant improvements in process economics and also serve to prevent unwanted resource loss. It is desirable to recover more than 80 vol %, preferably more than 90 vol %, of the methane when detrimental impurities are removed.
Synthesis gas (syngas) also typically requires removal and separation of various components before it can be used in fuel, chemical and power applications because all of these applications have a specification of the exact composition of the syngas required for the process. As produced, syngas can contain at least CO and H2. Other molecular components in syngas can be CH4, CO2, H2S, H2O, N2, and combinations thereof. Minority (or trace) components in the gas can include hydrocarbons, NH3, NOx, and the like, and combinations thereof. In almost all applications, most of the H2S should typically be removed from the syngas before it can be used, and, in many applications, it can be desirable to remove much of the CO2.
Additionally, separation of noble gases, such as xenon and krypton, can present an economic opportunity, but moreover, separation be required to avoid environmental hazard. For example, various nuclear processes may produce radioactive xenon and/or krypton, which cannot be safely vented to the atmosphere due to radiation hazard, and therefore, must be isolated or stored for a required period so as to be rendered harmless. Furthermore, these noble gases can be present in very low concentrations with other components; thus, selective separation is necessary.
Adsorptive gas separation techniques are common in various industries using solid sorbent materials such as activated charcoal or a porous solid oxide such as alumina, silica-alumina, silica, or a crystalline zeolite. Adsorptive separation may be achieved by equilibrium or kinetic mechanisms. A large majority of processes operate through the equilibrium adsorption of the gas mixture where the adsorptive selectivity is primarily based upon differential equilibrium uptake of one or more species based on parameters such as pore size of the adsorbent. Kinetically based separation involves differences in the diffusion rates of different components of the gas mixture and allows different species to be separated regardless of similar equilibrium adsorption parameters.
Kinetically based separation processes may be operated as pressure swing adsorption (PSA), temperature swing adsorption (TSA), partial pressure swing or displacement purge adsorption (PPSA) or as hybrid processes comprised of components of several of these processes. These swing adsorption processes can be conducted with rapid cycles, in which case they are referred to as rapid cycle thermal swing adsorption (RCTSA), rapid cycle pressure swing adsorption (RCPSA), and rapid cycle partial pressure swing or displacement purge adsorption (RCPPSA) technologies, with the term “swing adsorption” taken to include all of these processes and combinations of them.
Typically, zeolite adsorbents used in such gas separation processes may either have good kinetic separation selectivity for the contaminant or high capacity for the contaminant, but not both. Furthermore, the zeolite adsorbents may not have desirable surface properties, such as suitable hydrophobicity.
Thus, there is a need to provide additional adsorbent materials, which can be synthesized to have a combination of desirable functionalities including improved adsorption capacity and selectivity as well as adjustable hydrophobicity.